1. Field of the Invention
The invention pertains to a method and a device for controlling telecommunication connections via digital switching networks, in particular for telephone connections between participants of a telephone network like the integrated-services digital network (ISDN).
2. Prior Art
A frequently observed scenario of telecommunication via switching networks is that a caller is connected to an answering machine, although he intended to talk directly to a human individual. So the caller hangs up and pays for a not desired telephone connection. Currently every time telephone participants talk to an answering machine they have to pay telephone charges and waste time.
Another typical scenario in the field of telephone communication is where a node is shared by more than one individual and that a caller wants to talk to a definite person who currently is not present. Thus, another person will take the call, accordingly causing costs and time.
A first prior art approach for preventing non-desired telephone calls is an automatic telephone answering machine, disclosed in Japanese Patent Publication JP 60-229561, assigned to NEC Corporation and entitled "Automatic Telephone Answering Device". Its purpose is that the answering device automatically calls a called subscriber only when a calling subscriber voices a specific word, but does not answer to a silent telephone call. Proposed is a responding message sender controlled by the output of a voice detector which detects the specific speech of the calling subscriber and sends out a control signal to a speech answering circuit and a calling circuit. Both circuits are activated and the speech answering device sends out an answer message to the calling subscriber and also transmits a tone to let the subscriber know of the incoming call.
A second known approach, a responder absence indicating system for telephone sets, is disclosed in Japanese Patent Publication JP 59-122066, assigned to NEC Corporation and entitled "Display System for Absence of Responsor from Telephone Set". Proposed is to store an absent status by identifying a dial number of a registered absent status. The objective is to allow an originating subscriber to confirm absence of a responsor by storing presence or absence of the responsor regarding every telephone set. It is particularly proposed to discriminate and display a dial number for absence registration from a telephone set. A controller receives a dial number of a telephone set and sends it to a discriminating circuit which discriminated the number to store the absence state in the address of said telephone set. When a second telephone set originates to the first telephone set, a tone is sent out and the absence of the responsor from the first telephone set is confirmed.
There exist further approaches which let a called subscriber know who calls, to identify a caller before answering. In a respective system disclosed in Japanese Patent JP 1243757, assigned to Toshiba KK, the ISDN subaddress information is utilized for that purpose.
A similar approach for identifying a calling terminal is subject matter of European Patent Application 0 680 239, assigned to Nippon Telegraph and Sharp KK. There, a connecting facility connects calls from an identified calling terminal to a private ISDN terminal. A switching system has an identifier unit which identifies the calling terminal of an incoming call by a calling number and a subaddress in a call-setting message. Finally, an approach for improving communication effectiveness and efficiency by excluding unnecessary connections of terminals is disclosed in Japanese Patent JP 7123170, assigned to Nippon Telegraph, where an incoming call from a terminal to another terminal, which are interconnected via ISDN, is notified to a user through a display unit. An address circuit hereby sends a predefined specific code to a substitute address information element at the time of transmission of the call. An address checking circuit checks whether the address is available in the incoming call signal. If the address is not available in the calling signal, a control circuit identifies it as standard communication mode and notifies this to the user through the display unit.